Changing HDRI exposure within Blender
Given the answer of troy_s in this question here How to measure the dynamic range of an HDRi?, he states in the first paragraph that:


  An F-Stop is a halving and doubling of light, relative to the exposure
  referenced.


Now from my own trial and error testing in Blender, that's exactly what happens when you change the exposure value in Blenders color management settings:



If we now apply this thinking as a formula, for each color pixel this would mean to do the following for each color channel:

Pixel(x) = x * math.pow(exposure_value, 2)


Transferred into nodes, this could look like the following Node setup:



In a world node tree, you would now have something as easy as this if you transfer it into a node group:



You can find this node group, and also a similar one for the compositor to doodle around, in this Blend: